mem: Add callback to compute stats prior to dump event

mem: Add callback to compute stats prior to dump event

The per rank statistics are periodically updated based on
state transition and refresh events.

Add a method to update these when a dump event occurs to
ensure they reflect accurate values.
Specifically, need to ensure that the low-power state
durations, power, and energy are logged correctly.

Change-Id: Ib642a6668340de8f494a608bb34982e58ba7f1eb
Reviewed-by: Radhika Jagtap <radhika.jagtap@arm.com>